1. Field of the Invention
The present invention relates to optical bench subassemblies, particularly optical fiber subassemblies based on optical benches, and more particularly wavelength multiplexer/demultiplexer (MUX/DEMUX) modules/subassemblies based on optical benches.
2. Description of Related Art
There are many advantages of transmitting light signal via optical fiber waveguides and the use thereof is diverse. Single or multiple fiber waveguides may be used simply for transmitting visible light to a remote location. Complex telephony and data communication systems may transmit multiple specific optical signals. The data communication systems involve devices that couple fibers in an end-to-end relationship, including optoelectronic or photonic devices that include optical and electronic components that source, detect and/or control light, converting between light signals and electrical signals, to achieve high speed and high capacity data communication capabilities.
The fiber optics communication networks continue to demand increasing bandwidths and flexibility to different communication protocols. Wavelength division multiplexing (WDM) is an exemplary technology that puts data from different sources together on an optical fiber with each signal carried at the same time on its own separate light wavelength. Using the WDM system, separate wavelengths or channels of data can be multiplexed into a light stream transmitted on a single optical fiber. There can be coarse (CWDM) and dense (DWDM). Often the WDM devices use thin-film bandpass filters and mirrors as part of the optical system doing the wavelength based optical signal splitting. Instead of or in addition to thin film filters, prisms and arrayed waveguides (sometimes called phased arrays) are used.
From a terminology viewpoint, a device that multiplexes different wavelength channels or groups of channels into one fiber is a multiplexer, and a device that divides the multiplexed channels or groups of channels into individual or subgroups of channels is a demultiplexer. Specifically, a multiplexer combines several channels of optical signals into a single signal, or in reverse, a demultiplexer separates a single multichannel signal into several individual channel signals, such devices are referred to as a multiplexing or demultiplexing module, or simply multiplexer or demultiplexer.
Multiplexers/De-multiplexers (Mux/DeMux) are needed in optical modules such as quad small-form-factor pluggable (QSFP) transceivers. The QSFP is a full-duplex optical module with four independent transmit and receive channels. It is designed to replace four single-channel small-form-factor pluggable (SFP) and in a package only about 30% larger than the standard SFP. To equip a QSFP and similar transceivers requiring multiple wavelengths, a small Mux and DeMuxdevice is very important. Accordingly, there is a great need for such optical modules being made small, and at the same time, the modules so designed are amenable to small footprint, broad operating wavelength range, enhanced impact performance, lower cost, and easier manufacturing process.
In a prior art thin-film filter-based multiplexer, light of one wavelength per fiber is launched through a collimating lenslet and its associated bandpass filter directly above it, bouncing between the filters that reflect all but the transmission wavelength, and a reflector plane. The combined beam is reflected through an exit lenslet and focused for coupling into a single fiber. For example, a MUX/DEMUX for multi-mode fiber uses a molded optic, and employs both thin-film filters and a lens array.
U.S. Pat. No. 8,488,244 discloses designs of optical devices providing multiplexing or demultiplexing functions. According to one disclosed embodiment, an optical device or an assembly employs an array of micro lenses, an array of filters and a glass block all bonded onto a substrate to provide multiplexing or demultiplexing functions. To compensate for possible errors caused by some or all of these components, one or more compensatory optical plates are provided to respectively correct these errors. Depending on implementation, the compensatory optical plates may be designed differently to correct various errors.
FIG. 1 shows an exemplary configuration 300 of a Mux/DeMux assembly according to one embodiment disclosed in U.S. Pat. No. 8,488,244. One of the disclosed benefits, advantages and objectives of the present invention is to provide such an optical device with the size and functionalities for small form factor transceivers such as QSFP transceivers.
According to the disclosure in U.S. Pat. No. 8,488,244, and as shown in FIG. 1, all major components such as a collimator 302, a glass block 304 and a micro-lens array 306 are bonded to a substrate 308. As a result, at least two distinctive features are shown in comparison to the prior art: 1) channels on one side of the device are used with filters on the other side replaced by a high-reflectance coating which reflects light with all wavelengths; 2) channel collimators were replaced by a micro-lens array. In this design, a collimated beam bounces twice before reaching the next channel. When using as a DeMux, after passing the filter, the light beam for each channel is then focused by a micro-lens with a receiver located at or around the focal point of the micro-lens. The device can be used as Mux or DeMux with transmitter/receiver array pitch matches with the pitch of the micro-lens. The convex side of micro-lens can face either a filter array or a transmitter/receiver array.
In operation, a light beam is projected into the collimating lens 302. A segment anti-reflective coating 312 on the glass block 304 transmits the light beam through the glass block 304. The light beam hits the filter array 310 that includes four filters, each is made or configured to allow one specified wavelength to pass through and reflects others. A first filter in the filter array 310 allows a wavelength to transmit through. The transmitted wavelength is projected into the micro-lens array 306. A corresponding lens on the micro-lens array 306 couples the transmitted wavelength out to a receiver. Depending on application, an array of electronic devices 316 may be a laser diode (LD), GaAs PIN photodiode or other type of device to receive the transmitted wavelength (signal) or to transmit one or more signals into the assembly 300.
Meanwhile, the first filter in the filter array 310 reflects other wavelengths. The reflected wavelengths transmit in the glass block 304 and hit a high-reflection (HR) coating on the glass block 304 that reflects the reflected wavelengths back to a second filter in the filter array 310. Similar to the first filter, the second filter transmits one wavelength and reflects all others. The transmitted wavelength goes through a corresponding lens on the micro-lens array 306 to couple the transmitted wavelength (signal) out of the assembly 300. The reflected wavelengths from the second filter continue along the remaining filters in the filter array 310 and are eventually separated and coupled out through the lens on the micro-lens array 306.
in addition to the thin-film filter block described above, other prior art constructs multiplexers and demultipliexers with prisms, arrayed waveguides (AWGs), or diffraction gratings. The prism based units rely on a transmissive material's index of refraction dispersion to spatially separate the wavelengths, and to get sufficient separation, the prisms become undesirably large, increasing weight and cost. The AWG operates on the phased array principle; they are very temperature sensitive, and they are inherently lossy (˜3 dB) because there cannot be an infinite number of waveguides between the free-space regions, combined with the loss of injecting light from laser sources or fiber into the thin guides. The diffraction gratings are also inherently lossy, offering the highest diffraction efficiencies at only one angle or wavelength. Optical alignment between the discrete components in all the types of multiplexers is an important factor, where the AWG has the advantage that the number of components involved is more limited, since the multiplexing region is entirely photolithographically defined.
For example, the subassembly disclosed in U.S. Pat. No. 8,488,244 uses a substrate on which various components are accurately affixed to obtain acceptable optical alignment. The separate components (e.g., lens, etc.) are required to be accurately aligned and affixed to the substrate at tight tolerances, which involve challenging manufacturing steps. Specifically, various components must be assembled on the optical bench with optical alignment of the various optical components with sub-micron precision, in order to achieve the overall precision required to couple optical signals between the input and output of the Mux/Demux (i.e., from source to receiver). Optical alignment is more critical for single-mode operation, since most of or all of the optical components being used must be actively (manually) aligned to get good coupling of the light from source to receiver. This increases production/assembly costs and reduces yield, in addition to the challenges of achieving acceptable precision levels.
In the past, attempts were made to provide an injection molded polymer optical bench, which includes molded optics, and which could be coated with metallized thin film deposition to create reflective optics on the polymer optical bench. However, the tolerance of such polymer optical benches cannot meet the requirements for single-mode optical communications in a reliable and cost-effective manner.
What is needed is an improved subassembly for Mux/Demux, which improves manufacturability, throughput, tolerance, ease of use, functionality and reliability at reduced costs.